The present invention relates to a process for preparing a superabsorbent roll good for use in disposable absorbent hygiene articles such as diapers and sanitary napkins and to the products of the process. A water-based binder, preferably foamed, is combined with conventional superabsorbent particles to form a discrete but integral superabsorbent layer on the surface of a fibrous absorbent structure. When used in disposable absorbent products, the integral superabsorbent layer is located away from the intended fluid insult. The insult side of such disposable absorbent product preferably contains an acquisition layer of bonded stiffened cellulosic or synthetic fibers bonded with thermoplastic binder fibers or powder, a latex binder, or a combination thereof.
Conventional absorbent articles such as baby diapers, adult incontinence devices, and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet whose function is to allow the unobstructed passage of fluid to the absorbent core, and a liquid impervious backing sheet usually of plastic material, whose function is to contain the absorbed fluid and prevent it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article.
The absorbent core of these absorbent articles is typically constructed of defiberized wood pulp with or without superabsorbent polymer granules. The absorbent core is typically formed on a pad-forming unit of a converting machine on a carrier tissue to facilitate processing. Some absorbent core-forming units are equipped with layering capability in which a second discrete fluff layer may be laid over a primary fluff-based absorbent layer to form a multi-layer absorbent structure. In these absorbent structures, the primary layer may include superabsorbent polymer granules. With regard to conventionally produced absorbent structures, reference is made to U.S. Pat. Nos. 5,009,650, 5,378,528, 5,128,082, 5,607,414, 5,147,343, 5,149,335, 5,522,810, 5,041,104, 5,176,668, 5,389,181 and 4,596,567, the disclosures of which are hereby incorporated herein by reference.
In recent years, market demand for thinner and more comfortable absorbent articles has increased. Ultra-thin feminine napkins are no longer constructed from loose wood pulp, which tends to give a bulky product, but with roll good based airlaid absorbent cores in which a roll of preformed absorbent core material is unwound directly onto the absorbent pad-making machine without the defiberization step required for fluff-based products. The roll-goods based approach results in a product thinness not possible by loose fluff-based technology.
With respect to the superabsorbent polymer component of absorbent structures, it is known in the art (e.g. U.S. Pat. Nos. 3,669,103 and 3,670,731) that carboxylic polyelectrolytes may be cross-linked to form materials commonly referred to as superabsorbents. These materials are used to enhance the absorbency of disposable absorbent articles. Superabsorbent polymer may be coated (continuous or discontinuous coatings) on various materials (see U.S. Pat. No. 4,076,673). A layer of superabsorbent particles can be bound to a fibrous substrate using the inherent tackiness of a water swollen superabsorbent as described in U.S. Pat. No. 3,686,024. Thermally-activated adhesive binders may also be used as described in EP 0 641 835 A1. U.S. Pat. No. 5,128,082 teaches the fabrication of a superabsorbent-containing web in which the superabsorbent granules are deposited between two layers of fiber, with a latex bonding agent applied to each side of the sandwich structure to stabilize the web and bind the granules. U.S. Pat. No. 5,071,681 discloses preparing an airlaid fibrous web and applying a water-insoluble binder to one surface and a water soluble polymer capable of forming a superabsorbent to the other surface of the web. In U.S. Pat. No. 4,444,830, liquid superabsorbent precursor solution is chemically foamed and applied to a base fluffing material and the coated fluffing material is dried, disintegrated and mechanically worked into a fibrous fluff matrix which contains absorbent polymer platelets distributed throughout the matrix.
Adhering highly water-swollen superabsorbent granules to fibrous substrates as described for example in U.S. Pat. No. 3,686,024 causes a processing disadvantage because a considerable amount of water must be removed from the absorbent structure prior to use. Thermally bonding the particles to the fibers in a web entails a separate processing step of combining the adhesive with the granules before they are spread onto or mixed with the fibrous substrate and heated to fuse the thermal adhesive. Spraying a dilute solution of liquid superabsorbent precursor onto an airlaid web causes superabsorbent penetration into the web so that the superabsorbent polymer is less a discrete layer than a coating on the fibers of the web. Adding superabsorbent granules to a web between two or more fiber distributor heads and spraying with latex bonding agent embeds the granules in the web and precludes formation of a discrete layer of superabsorbent on the bottom of an airlaid web. It is recognized by those skilled in the art, that when superabsorbent particles are mixed with fibers, either bonded or unbonded, there is a competition for fluid between the superabsorbent and the surrounding fibrous structure which prevents the superabsorbent from achieving its full absorption potential. The present invention avoids these limitations by localizing superabsorbent particles to a discrete but integral layer.
Applicants have now discovered that it is desirable to bond a distinct layer of superabsorbent granules to one side of an absorbent structure.